In the applications mentioned above, glasses made of transparent plastics material must be treated in such a way as to make them insensitive, or at the very least highly resistant, to various kinds of aggressive outside influences. Such influences include mechanical attack resulting from the collision of various kinds of solid particles with the glass. This gives rise to abrasion of the plastics material of the glass, which leads to degradation of its light transmitting qualities; it also leads to the creation of parasitic diffraction.
Detrimental outside influences also include chemical attack due to various factors which are capable of causing chemical reaction with the material of which the glass is made. Such chemical reactions may be caused, for example, by the humidity of the atmosphere, or by various kinds of atmospheric pollution, or by washing liquids, or by contact with oil or fuel. Chemical changes in the plastics material of the glass may also be caused by the effects of exposure to some kinds of radiation, such as ultraviolet radiation.
These chemical reactions have the result of altering the appearance of the glass, and also result in deterioration of its light transmitting capability.
In order to overcome the above mentioned drawbacks, it has previously been proposed to protect the glasses of plastics material by means of a protective coating. These protective coatings can be classified in two categories in accordance with the way in which they act.
The first of these categories consists of hard, abrasion-resistant, varnishes or lacquers, which form a transparent screen that protects the glass of plastics material against any mechanical and chemical attack. The lacquers in this category are in general terms made from organo-silicone or acrylic compounds, the resistance of which to abrasion and chemical attack is nearly as good as that of vitreous glass. However, these lacquers have the disadvantage that they have a different coefficient of expansion from that of the plastics material of the glass which they are intended to protect, so that it can be necessary to provide, between the lacquer and the plastics material of the glass itself, an intermediate layer which is capable of absorbing the mechanical stresses that can be produced during temperature variations--as is indeed the case with motor vehicle headlamp glasses.
The lacquers or varnishes in the second category are those known as self-healing lacquers, which have the ability to re-form, by themselves, a perfect surface after having been subjected to mechanical attack such as scratching. Protective coatings of self-healing lacquers are in general made from polyurethane based compositions. This is for example the case in the coating which is described in French published patent specification FR 2 677 657A, filed in the name of Valeo Vision, which includes a coating composition formed from a reticulated polyurethane-polyurea.
Nevertheless, it has been found that the adhesion of the protective coating on the plastics material of the glass was insufficient, in spite of the fact that such methods for treating the surface of the glass were used as corona, flame, and plasma microwave treatment, before the protective coating was deposited on the glass. This is found to give rise to rapid deterioration in the optical transmission characteristics of the glass, so making it unsuitable for its purpose.